1. Field of the Invention
The present invention concerns a method for acquiring data from an examination region with a magnetic resonance apparatus, the examination region being larger than a maximum data acquisition region of the magnetic resonance apparatus.
2. Description of the Prior Art
Magnetic resonance (MR) technology enables non-invasive medical imaging of a subject. A region of a patient to be examined in a basic magnetic field is exposed to radio-frequency magnetic energy (RF field) for excitation of an emission of MR signals. For spatially resolved imaging, the MR signals are detected, and spatial coding is achieved using spatially varying gradient fields. A sequence of gradient fields and RF fields for data acquisition is designated as a measurement sequence. The quality of an MR exposure depends on, among other things, the homogeneity of the basic magnetic field. This is typically generated with a superconducting basic field magnet and, together with requirements of the gradient and RF field, determines a usable maximum data acquisition region of the MR apparatus. This is typically in the range of a few decimeters. The requirements for the magnetic and RF fields, for example with regard to spatial and temporal resolution, depend on the respective measurement sequence to be implemented, such that the maximum data acquisition region can be varied in its dimensions depending on the measurement sequence, i.e. depending on the intended imaging.
Acquiring data from an examination region that is larger than the available maximal acquisition region of the magnetic resonance apparatus is problematical. This problem is particularly serious in MR apparatuses with a so-called short bore, i.e. with a short basic field magnet. Examples of problematic examinations are whole-body examinations, examinations of the spinal column along its entire length, peripheral angiography examinations and precautionary screening examinations with regard to metastases.
The advantages of a short basic field magnet, namely allowing better access to the patient for interventional procedures and improving patient comfort by alleviating claustrophobic feelings, are offset by the small homogeneity volume of the associated basic magnetic field. It is a goal in MR technology to generate optimally large, homogeneously exposed MR images in an optimally short examination time. The handling of the magnetic resonance apparatus should be simpler rather than more complicated from the viewpoint of the user.
Two approaches are pursued for acquisition of body regions exceeding the maximum data acquisition region of the MR apparatus: namely “Step-by-Step&Compose” technique and the “Move-during-Scan” technique.
In the “Step-by-Step&Compose” technique, a user separates the examination region into a number of sub-regions that are individually optimally measured isocentrically, i.e. centrally in the maximal acquisition region by movement of the patient bed through successive stationary bed positions. A completed data set for an MR image is obtained in each measurement. The various MR exposures are merged in a subsequent post-processing step. A difficulty of this technique is that numerous measurement parameters of the various partial measurements must be perfectly matched to one another by the user. This involves, for example:    the spatial position and size of the acquisition regions of the various, possibly partially overlapping, partial measurements,    the optimal temporal matching of the table shifting steps, for example for contrast agent angiography,    the selection of the respective coils to be used,    the accounting for the homogeneity and linearity of basic magnetic, gradient magnetic and RF fields for homogeneous exposure of the MR acquisitions and to reduce distortions.
The user must account for many dependencies in planning and post-processing of such an examination. This requires a large time expenditure as well as significant expertise.
In the “Move-during-Scan” technique, a three-dimensional image data set is generated while a patient is continuously moved through the MR apparatus. The speed of the table and the frequency of the line-by-line excitation and scanning of the MR signals are thereby matched to one another such that the necessary spatial resolution results along each axis. This technique is still in its infancy and exhibits the disadvantage of being limited with regard to the imaging methods (i.e. measurement sequences) for which it can be used.
Neither known technique is a satisfactory solution for the problem described above with regard to the measurement preparation time, the operating comfort and measurement result.